Paramyxoviruses are enveloped viruses that enter cells by fusing directly with the cell membrane. During entry the two viral surface glycoproteins HN (the receptor-binding molecule) and F (the viral fusion protein) cooperate in a highly specific way to mediate fusion upon receptor binding. However, during virus replication and assembly in the cytoplasm the fusion process is silent, and until an incoming virus meets its target cell, the fusion machine is inactive. How is the fusion process coordinated in time and place? To understand these mechanisms and elucidate how paramyxoviruses enter cells, we study the human parainfluenza virus (HPIV), an important cause of respiratory disease in children. Our results have uncovered fundamental roles of the receptor binding protein in paramyxovirus fusion, and principles of coordinated interaction between the glycoproteins during the viral life cycle. We have developed new, interdisciplinary tools to dissect the complex process whereby paramyxoviruses control entry, and central questions in the field can now be addressed. Specific Aim 1. Mechanisms by which the diverse functions of the paramyxovirus receptor-binding molecule are coordinated. How does attachment by the primary receptor binding site of HN translate into the F-triggering that is promoted by HN's second binding/triggering site? Mutations at HN's two functional binding sites will be used to study how HN works to trigger F. These experiments will define how the activities of HN are coordinated for HPIV, and how distinct strategies in other paramyxoviruses accomplish the same ends. Specific Aim 2. How the receptor-binding molecule and the fusion molecule communicate during fusion and entry. What is the nature of the communication between HN and F during fusion and entry, and how is this communication regulated? A series of innovative strategies will test the hypothesis that specific HN-F interactions regulate F-activation. These experiments will for the first time define the biologically relevant dialog between the molecules that comprise the paramyxovirus fusion machine. Specific Aim 3. Regulation of fusion during the paramyxoviral life cycle in airway cells. Correct timing of F-activation is essential for entry;for infection, triggering must occur only when F is in contact with the target cell membrane. How are the diverse functions of these proteins regulated during the viral life cycle, and can we subvert this regulation as a strategy for antivirals? We will test the hypothesis that dysregulation of F-triggering precludes successful infection, in a system that represents human lung epithelium. We will determine whether compounds can prematurely trigger F, and whether this may prevent airway infection, providing a new antiviral strategy. The underlying principles revealed here will offer insights into complex molecular machines that require allosteric activation. The results will likely apply to other pathogens that must trigger these key entry activities only at a specific time and place to initiate infection. PUBLIC HEALTH RELEVANCE: This research proposal will lead to a better understanding of how paramyxoviruses enter human cells to initiate infection. Newly identified mechanisms involved in the entry process could serve as potential new targets for antivirals to treat or prevent human respiratory diseases.